Process for epitaxial growth of semiconductor single crystals



A. REISMAN Jan.,1o, 1967 PROCESS FOR EITAXIAL GROWTH OF SEMICONDUCTOR SINGLE CRYSTALS Filed DSC. 3l, 1965 ARNOLD REISMAN BY MQW ATTORNEY United States Patent O PROCESS FOR EPITAXlAL GROWTH OF SENE- CONDUCTGR SINGLE CRYSTALS Arnold Reisman, Yorktown Heights, N .Y., assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Dec. 31, 1963, Ser. No. 334,859

15 Claims. (Cl. 148-175) This invention relates to a process for epitaxially growing Si and Ge semiconductor single crystals via a polyhalide reduction process. More particularly, depending on how the process is eiected the reducible polyhalides are either unstable at room temperature, disproportionating with the liberation of the condensed semiconductor, or present as vaporous species.

Previously employed processes for the single crystal growth of Ge and Si can be categorized as either proceeding via disproportionation mechanisms or via reduction processes. More specically, compounds such as GeX2 where X is a halogen (e.g., C12, I2, Br2) formed at elevated temperatures are caused to disproportionate 'at some lower temperature, one of the disproportionation products being the desired semiconductor. Alternately, a compound such as GeX4 where X is usually C12 but may be I2 or Br2, which compound is stable at room temperature is volatilized and impinged on a single crystal at elevated temperatures in the presence of H2 resulting in the reduction of the tetrahalide and simultaneous deposition of the desired semiconductor. While both of the generalized processes described, namely, the disproportionation of :a species formed at high temperature at some lower temperature, or the reduction of a vapor phase species at elevated temperature have been used extensively in the growth of semiconductor single -crystals from the vapor, neither approach fulfills all of the requirements one would like to see present in a vapor growth process. For example, taking Ge as a case in point, the most extensively employed disproportionation process involves reacting iodine with a Ge source at elevated temperatures (40()- 600 C.) forming a mixture of Gel., and Gel2 in the vapor phase. This vapor phase is then carried `over a substrate at some lower temperature in the range 30G-400 C. at which point the GeI2 species disproportionates to form Ge which deposits, and Gel4 as a byproduct. The range of operation for this process is limited and the rate of achievable growth consistent with obtaining high quality deposited crystals is low, andin addition, the process is ineicient as normally employed. An alternate method for growing Ge single crystals involves the transport of GeCl4 in a hydrogen stream over a single crystal substrate heated to between 75Q-920 C. The GeCl4 is reduced for-ming Ge as one of the products; the Ge depositing upon the heated single lcrystal substrate. While this process is able to yield high quality single crystal growths .at relatively rapid rates (0.5-1.0 rnicron per minute) the quality of the growths becomes difficult to control when the deposit achieves thicknesses much in excess of ll5 microns. Furthermore, because of the fact that concurrent with the reduction of the tetrahalide, the latter is reacting with the heated substrate to form the dihalide, and because of the lfact that diffusion of impurities from the substrate into the depositing ylayer becomes rapid Iat the elevated temperatures required, the impurity gradient across the junction formed between the substrate and the deposited layer is not controllable and is not abrupt. Finally, in comparing the two processes, one via a disproportionation and the `other via a reduction mechanism (the latter being attended by an undesirable substrate etching reaction) it is realized that the range of operation of each is limited, and that between Patented Jan. l0, 1967 ice the operable ranges of both, there exists a large temperature interval which offers potential for trading olf the advantages of each process and which 'at present is not usable.

An object of this invention is a process for epitaxially depositing Ge via polyhalide reduction process in the temperature interval SOO-920 C.

A further object of this invention is a process for epitaxially depositing Ge via a reduction of an in situ generated non-disproportionable Ge polyhalide.

Another object `of this invention is to `deposit Ge Via a reduction process from a vapor phase non-disproportionatable species which does not react appreciably with the substrate upon which Ge is to be deposited.

A further object lof this invention is a process for epitaxially depositing Ge via a reduction process from a vapor phase disproportionatable species which does not react appreciably with the substrate upon which the Ge is to be deposited.

Still another object of the invention is a process for epitaxially depositing Si via polyhalide reduction process in the temperature interval l0501250 C.

' It is an object of this invention to define a process which makes possible the formation of :more abrupt single crystal junctions via a vapor phase epitaxial process due to the minimization of substrate etchant reactions which normally accompany the desired growth processes.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawmg.

In the drawing:

The `figure is a cross-sectional representation of the reaction train in which the epitaxial deposition of Ge or Si is eiiected.

The process of the invention utilizes as a source of reducible polyhalides of Si or Ge the products of the reactions of either a Si or Ge halide 4or pure halide or halide acid carried either in a helium yor H2 stream and a Ge or Si reaction column. The products of the lreaction lare diierent depending upon whether the source semiconductor halide is reacted with the packed columns in the presence `of He or H2, the former giving rise to disproportionatable, reducible polyhalides, and the latter giving rise to nondisproportionat-able reducible polyhalides. The reducible, disproportionatable or nondisproportionatable eiuent polyhalides are impinged on a semiconductor substrate heated to a temperature necessary to cause reduction to occur, resulting in the deposition of the semiconductor epitaxially upon the substrate. Because the reducible disproportionatable or nondisproportionatable polyhalides are in a state of equilibrium relative to a condensed Ge or Si phase when etlluing from the reaction column, they do not tend to react with the substrate, they tend only to be reduced. Consequently, substrate etching and incorporation of the etch products into the growing layer is greatly minimized. Furthermore, the temperature necessary for the reduction of the eluing Ge polyhalides is in general lower than that required for conventionally employed materials. p

The epitaxial layers of Si or Ge grown by Iany of the procedures constituting the process of the invention, find utility in the fabrication of transistor or diode structure which may be employed in computer llogic circuits or communication equipment (such as radios).

The following alternative .procedures can be employed for epitaxially growing Ge or Si.

Procedure ,eh-Growth from dispropo-rtionatable reducible eiuents.

He from a puried source 1 is carried through a volatile Ge or Si halide or pure halogen source 2 prefefably sich, GBCLL, C12, I2, Brz, GeBr4, Gelb SiBfb The saturated gas stream containing the semiconductor halide or pure halogen emanating from the source 2 is carried through the Si or Ge packed heated bed 3 e.g. Ge: 290450 C.; Si: 70D-980 C. The equilibrium products efuing from said bed through a gas disperser in inner nozzle 4 are intermixed with purified H2 emanating from a H2 source 5 which enters a reaction chamber 7 at the outer nozzle 6. The gases emanating from the nozzles 4 and 6 intermix providing a semiconductor content in the vapor phase of between .25 and 1.0 mole fraction. The intermixed gases are then impinged on the heated substrate 8 supported on the Si or Ge R. F. heated susceptor pedestal 9 e.g. both heated in the case of Ge to 50G-920 C. and in the case of Si t-o 1050-1250 C. Reduction of the semiconductor vapor phase species occurs in the region of the substrate and pedestal, the former serving as a seed for epitaxial growth from the vapor of Si or Ge resulting from the reduction process. The packed bed of Si or Ge, and the reaction chamber are heated by the resistance winding 11 and the substrate and pedestal are heated to a somewhat higher temperature by the radio frequency coil 12. The temperature of the pedestal is monitored by a thermocouple contained in the thermocouple well 14 and the Volatile reaction products are exhausted via the vent 13.

Procedure B.-Growth from nondisproportionatable reducible efuent.

With the He valve 1 closed, H2 from the H2 source S is carried simultaneously through the halogen source 2, the heated packed bed 3 and the nozzle 6 into the reaction chamber. The remaining procedural operations are the same as described in the preceding section.

Using either of the above procedures and a semiconductor conc. of 0.5 mole percent at total ow rate l liter per minute the growth rates for Ge and Si will range between .l-.5 micron per minute on a substrate of 0.5 diameter.

EXAMPLE I Formation of dsproportionatable reducible products He is carried through a source of GeCl4 and then through a packed bed of Ge heated to 300"A C. The effluent gas from this bed is intermixed with a hydrogen stream and is permitted to come int-o contact with an unheated Ge pedestal and Ge substrate. A deposit of polycrystalline Ge coats the substrate. If the packed bed, reaction chamber, pedestal and substrate are all maintained at 300 C., the euent from the packed bed neither etches nor deposits Ge on the Ge substrate, demonstrating that the effluent will not etch the substrate of Ge and that the gas passing through the packed bed comes into equilibrium with this bed insofar as Ge content of the Vapor is concerned. At the exhaust portion of the reaction chamber Which is maintained at a temperature below 300 C., a mirror of Ge forms on the quartz wall of said chamber.

EXAMPLE II The process of Example I is repeated with the packed bed heated to 450 C. and the pedestal and substrate kept cold (i.e., at room temperature). A deposit of Ge again forms on the cool substrate. The procedure is repeated with the packed bed, reaction chamber, and pedestal all maintained at 450 C. The eluent from the packed bed neither etches the substrate nor deposits Ge upon it. As in Example I, a mirror of Ge forms in the vicinity of the exhaust portion of the apparatus. Thus, again it is shown that the eluent will not etch the Ge substrate.

EXAMPLE IH The process of Example I is repeated using a source of SiCl., and a Si bed heated at 800 C. and the Si pedestal and Si wafer (or substrate) at room temperature. A deposit of Si forms on the wafer (or substrate). With the Si bed, reaction chamber, pedestal and substrate all at 4 800 C., the eluent from the packed bed neither etches the substrate nor deposits Si upon it.

EXAMPLE 1V The process of Example I is repeated with the packed Si bed at 980 C. and the Si pedestal and Si wafer (or substrate) at room temperature. A deposit of Si forms on the wafer (or substrate). With the packed bed, reaction chamber, Si pedestal and Si Wafer (or substrate) at 980 C., the ei-liuent from the packed Si bed neither deposits Si on the substrate nor etches it.

EXAMPLE V The process `of Example I is repeated using SiCl4 as a source material and a packed Si bed maintained at room temperature and a Si pedestal and a Si single crystal substrate maintained at 800 C. In either case, the Si substrate is severely etched and a mirror of Si forms at the colder exhaust portion of the system. When the pedestal and substrate are maintained at a suiciently high enough temperature (1050-l250 C. to cause reduction of the SiCl.,= so as to deposit Si, this Si deposit becomes contaminated with the etchant products resulting in the formation of non-abrupt impurity concentration profiles across the grown junction.

B. The process of Example I is repeated using GeCl4 as a source material and a packed Ge bed maintained at room temperature and a Ge pedestal and Ge single crystal substrate maintained at 300 C. The Ge substrate is severely etched and a mirror of Ge forms at the colder exhaust portion of the system. When the pedestal and substrate are maintained at a suciently high enough temperature (500-920 C.) to cause reduction of the GeCl4 so as to deposit Ge, this Ge deposit becomes contaminated with the etchant products resulting in the formation of nonabrupt impurity concentration proles across the grown junction.

EXAMPLE VI Formation of nondsproportz'onatable reducible products H2 is carried through a source of GeCl4 and then through a packed bed of Ge heated to 300 C. The effluent gas from this bed is intermixed with a H2 stream and is permitted to come into contact with an unheated Ge pedestal and Ge substrate. No deposit of Ge coats this substrate nor is the substrate etched, nor is any condensation of any kind observed to form elsewhere in the system. It the packed bed, reaction chamber, pedestal and substrate are all maintained at 300 C., the eiuent from the packed bed neither etches nor deposits Ge on the Ge substrate demonstrating that the gas passing through the packed bed comes into equilibrium with this bed insofar as Ge content of the vapor is concerned. Thus, the Ge substrate is not etched.

EXAMPLE VH The process of Example VI is repeated with the packed bed of Ge heated to 450 C. and the Ge pedestal and Ge substrate maintained at room temperature. No deposit of Ge coats this substrate nor is the substrate etched nor is any condensation of any kind observed t0 form elsewhere in the system. If the packed bed, reaction chamber, pedestal and substrate are all maintained at 300 C., the efiuent from the packed bed neither etches nor deposits Ge on the Ge substrate demonstrating that the gas passing through the packed bed comes into equilibrium with this bed insofar as Ge content of the vapor is concerned. Thus, the Ge substrate is not etched.

EXAMPLE VIII The process of Example VI is repeated except that H2 is carried through .a source of SiCl4 and then through a packed bed of Si heated to 800 C. The effluent gas from this bed is intermixed with a H2 stream and is permittedto come into contact with an unheated Si pedestal and Si substrate. No deposit of Si coats this substrate nor is the substrate etched nor is any condensation of any kind observed to form elsewhere in the system. If the packed bed, reaction chamber, and pedestal are all maintained at 800 C., the effluent from the packed bed neither etches nor deposits Si on the Si substrate demonstrating that the gas passing through the packed bed comes into equilibrium with this bed insofar as Si content of the vapor is concerned. Thus, the Si substrate is not etched.

EXAMPLE 1X The process of Example VI is repeated except that H2 is carried through a source of SiCl., and then through a packed bed of Si heated to 980 C. The effluent gas from this bed is intermixed with .a H2 stream and is permitted to come into contact with an unheated Si pedestal and Si substrate. No deposit of Si coats this substrate nor is the substrate etched, nor is any condensation of any kind observed to form elsewhere in the system. If the packed bed, reaction chamber, pedestal and substrate are all maintained at 980 C., the eluent from the packed bed neither etches nor deposits Si on the Si substrate, demonstrating that the gas passing through the packed bed cornes into equilibrium with this bed insofar as Si content of the vapor is concerned. Thus, the Si substrate is not etched.

EXAMPLE X A. The process of Example VI is repeated using either SiCl4 as a source material, a packed Si bed maintained at room temperature and a Si pedestal and Si single crystal substrate maintained at 800 C. The Si single crystal substrate is severely etched and a mirror of Si forms at the colder exhaust portion of the system. When the pedestal and substrate are maintained at a suiciently high enough temperature (1050-1250 C.) to cause reduction of SiCl4 so as to deposit Si, this Si deposit becomes contaminated with the etchant products resulting in the formation of nonabrupt impurity concentration profiles across the grown junction.

B. The process of Example VI is repeated using GeCl4 as a source material, a packed Ge bed maintained at room temperature, and a Ge pedestal and Ge single crystal substrate maintained at 300 C. The Ge single crystal substrate is severely etched and a mirror of Ge forms at the colder exhaust portion of the system. When the pedestal and substrate are maintained at a suiciently high enough temperature (500-920 C.) to cause reduction of GeCl4 so as to deposit Ge, this Ge deposit becomes contaminated with the etchant products resulting in the formation of nonabrupt impurity concentration proiiles across the grown junction.

EXAMPLE XI He at a ow rate of 50 cc. per minute is carried through a source of GeCl4 maintained at 25 C. and then through a packed bed of Ge heated to 300 C. The effluent gas from this bed is intermixed with a H2 stream flowing at 950 cc. per minute Iand is permitted to come into contact with a Ge single crystal substrate supported on a Ge pedestal, the substrate and pedestal being heated to 500 C. The process is continued for one hour. At the conclusion of the process, an epitaxial deposit of Ge microns thick has grown on the single crystal substrate of Ge.

EXAMPLES XII-XVI The process of Example XI is repeated except that the packed `bed of Ge, the Ge substrate and the Ge pedestal are heated to the temperature set forth in Table I. The thickness of the epitaxial grown Ge deposit is also set forth.

TABLE I Packed Pedestal Deposit Example No. Bed C.) and Sub- (microns) strate C.)

EXAMPLE XVII He at a flow rate of 20 c-c. per minute is carried through a source of SiCl4 maintained at 25 C. and then through a packed bed of Si heated to 800 C. The efliuent gas from this bed is intermixcd with :a H2 stream iiowing at 80 cc. per minute and is permitted to come into contact with a Si single crystal substrate supported on a Si pedestal, the substrate and pedestal being heated to 1050 C. The process is continued for one hour. At the conclusion an epitaxial deposit of Si 16 microns thick has grown on the single crystal substrate of Si.

EXAMPLES XVIII-XXII The process of Example XVII is repeated except that the packed bed of Si, the Si substrate and the Si pedestal are heated to the temperature set forth in Table II. The thickness of the resulting epitaxial grown Si deposit is also set forth.

TABLE 1I Packed Substrate Example No. Bed C.) and Ped- Deposit estal C.)

soo 1, 250 17 850 1, 050 15 850 1, 250 19 980 1, 050 17 980 1, 250 19 EXAMPLE XXIII H2 at a iow rate of 50 cc. per minute is carried through a source of GeCl2 maintained at 25 C. and then through a packed bed of Ge heated to a temperature Iof 300 C. The effluent gas is inter-mixed with a H2 stream owing at 950 cc. per minute and is permitted to come into contact with a Ge single crystal substrate heated to 500 C. The process is continued for 1 hour. At the conclusion of the process, an epitaxial deposit of Ge 12 microns thick has grown on single crystal substrate of Ge.

EXAMPLES XXIV-XXXI The process of Example XXIII is repeated except that the packed bed of Ge, the Ge substrate and Ge pedestal are heated to the temperature set forth in Table III. The thickness of the resulting epitaxial grown Ge deposit is also set forth.

TABLE III Packed Pedestal Example No. Bed C.) of Sub- Thickness strate C.)

EXAMPLE XXXII H2 at a flow :rate of 20 cc. per -rninute is carried through a source of SiCl4 maintained at 25 C. and then EXAMPLES XXXIII-XXXVII The process in Example XXXII is repeated except that the packed ebed of Si, the Si substrate and the Si pedestal are heated to the temperature set forth in Table IV. The thickness of the resulting epitaxial lgrown Si deposit is also set forth.

TABLE IV Packed Pedestal Example No. Bed C.) of Sub- Thickness strate C.)

XXXIIL 800 1, 250 16 XXXIV 875 l, 050 13 875 1, 250 l 950 1, 050 16 950 1, 250 1S All of the epitaxial layers grown in the above examples may be suitably doped via the introduction of conventional gaseous doping agents into the reaction chamber enabling the formation of p-n, n-p, p+p, and n+-n structures.

Thus, the process of the invention provides a means lfor depositing epitaxial layers of Si or Ge through the use of disproportionatable reducible or nondisproportionatable reducible polyhalides of Ge and Si which because of the nature of their preparation are either unstable or vaporous in nature under normal -r-oom temperature conditions and which because of their method of preparation greatly minimize contamination of the growing epitaxial layer by the undesired etchant products which are a normal constituent of conventional Ge or Si reduction growth process.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A method of epitaxially depositing a semiconductor on a substrate comprising the steps of providing a bed of said semiconductor material, generating a non-etching semiconductor polyhalide vapor which is in equilibrium with said semiconductor material and reducing said polyhalide vapor to cause epitaxial deposition on said substrate.

2. A method according to claim 1 wherein the step of generating a semiconductor polyhalide vapor includes the step of passing a carrier gas selected from the group consisting of hydrogen and helium through a source of material selected from the group consisting of the halogens and a semiconductor halide to form a vapor of said gas and said material.

3. A method according to claim 2 further including the step of flowing said vapor of said gas and said material through said bed.

4. A method according to claim 1 wherein the step of reducing includes the step of intermixing a reducing gas with said semiconductor polyhalide vapor.

5. A method according to claim 1 wherein the step of :reducing includes the step of heating said semiconductor polyhalide Vapor with hydrogen to a temperature sucient to cause reduction of said polyhalide vapor,

6. A method according to claim 1 wherein said semiconductor material is silicon.

7. A method according to claim 1 wherein said semiconductor material is germanium.

8. A method of epitaxially depositing a semiconductor on a substrate comprising the steps of: generating a non-etching semiconductor polyhalide vapor which is in equilibrium with a source of said semiconductor material at a given temperature and reducing said semiconductor polyhalide at a temperature higher than said given temperature to cause epitaxial `deposition of said semiconductor on said substrate.

9. A process of epitaxially depositing a semiconductor material on a substrate which comprises the steps of:

( 1) Providing a carrier gas,

(2) Flowing said gas through a source of material selected from the group consisting of a halogen and a semiconductor tetrahalide to form a saturated gas stream;

(3) Passing said saturated stream through a bed of said semiconductor ymaterial heated to a temperature less than the temperature of said substrate and suficient to form a non-etching semiconductor polyhalide vapor which is in equilibrium with said semiconductor material;

(4) Introducing from a separate source a quantity of hydrogen so as to intermix with said polyhalide vapor in the region of said substrate thereby to reduce said polyhalide vapor and form an epitaxial deposition on said substrate.

10. The process of claim 9 wherein the semiconductor material is germanium and the gas is helium.

11. The process of claim 9 wherein the semiconductor material is germanium and the gas is hydrogen.

12. The process of claim 9 wherein the semiconductor material is silicon and the gas is hydrogen.

13. The process of claim 9 wherein the semiconductor material is silicon and the gas is helium.

14. The process of claim 9 wherein said semiconductor material is germanium and said bed is maintained yat a temperature of 290 C. to 450 C. and said substrate is maintained -at a temperature of 500 C. to 920 C.

15. The process of claim 9 wherein the semiconductor material is silicon and said bed is maintained at a temperature from 700 C. to 980 C. and said substrate is maintained at a temperature of 1050 C. to 1250o C.

References Cited by the Examiner UNITED STATES PATENTS 2,692,839 10/ 1954 Christensen et al. 148-175 2,763,581 9/1956 Freedman 148-175 2,767,052 10/1956 Hamer et a1. 23-87 3,009,834 11/1961 Hanlet 148-174 3,020,129 2/ 1962 Herrick 23-87 3,068,066 12/ 1962 Scheller et al 23-87 FOREIGN PATENTS 855,913 12/ 1960 Great Britain.

OTHER REFERENCES Glang et al.: Status of Vapor Growth in Semiconductor Technology, Metallurgy of Semiconductor Materials, vol. 15, August/September 1961, pages 27-43.

Matovich et al.: Surface Effects and Autodoping in Epitaxial Germanium Layers, September 29, 1961, pages 1-9.

DAVID L. RECK, Primary Examiner. N. F. MARKVA, Assistant Examiner. 

1. A METHOD OF EPITAXIALLY DEPOSITING A SEMICONDUCTOR ON A SUBSTRATE COMPRISING THE STEPS OF PROVIDING A BED OF SAID SEMICONDUCTOR MATERIAL, GENERATING A NON-ETCHING SEMICONDUCTOR POLYHALIDE VAPOR WHICH IS IN EQUILIBRIUM WITH SAID SEMICONDUCTOR MATERIAL AND REDUCING SAID POLYHALIDE VAPOR TO CAUSE EPITAXIAL DEPOSITION ON SAID SUBSTRATE. 